1. Field of Invention
The invention relates generally to optical aberration measurement and correction, and in particular to projection techniques in the objective measurement and correction of the human eye using a wavefront sensor.
2. Description of Background Art
There has been and continues to be a need to provide a person with improved visual acuity. Remodeling of the cornea using refractive laser surgery or intracorneal implants, adding synthetic lenses using intraocular lens implants or precision ground contact lenses or eye glasses provide known solutions. Further, it is known to correct vision astigmatically by surgical modification of myopic or hyperopic astigmatism through laser keratoplasty, keratomileusis, or photorefractive keratectomy. Laser sources are used to erode or ablate surfaces of the eye, typically reshaping the cornea. Prior to and during such surgery, precise measurements must be made to determine required surgical corrections.
The imprecise measurement technique of placing lenses of known refractive power anterior to the cornea and asking a patient which lens or lens combination provides the clearest vision has been improved with the use of autorefractometers, as described in U.S. Pat. No. 5,258,791 to Penny et al., or with the use of wavefront sensors as described by Liang et al. in xe2x80x9cObjective Measurement of Wave Aberrations of the Human Eye with the Use of a Hartmann-Shack Wave-Front Sensor,xe2x80x9d Journal of the Optical Society of America, Vol. 1, No. 7, July 1994, pp. 1949-1957, byway of example. Penny ""791 discloses the use of autorefractometer measurements for determining the appropriate corneal surface reshaping to provide emmetropia, a condition of a normal eye when parallel rays are focused exactly on the retina and vision is optimum. Spatially resolved refraction data, in combination with measured existing surface contour of the anterior surface of the eye, enable a calculation of a detailed spatially resolved new contour that provides corrected vision.
It would be an improvement in the art if such vision correction could be made without the need for these contour data, and further without the need for feedback from the patient regarding an appropriate lens. Liang et al. disclose the use of a Hartmann-Shack wavefront sensor to measure ocular aberrations by measuring the wavefront emerging from the eye by retinal reflection of a focused laser light spot on the retina""s fovea. A parallel beam of laser light passes through beam splitters and a lens pair that brings the beam to a focus point on the retina by the optics of the eye. Possible myopia or hyperopia of the tested eye is corrected by movement of a lens within the lens pair. The focused light on the fovea is then assumed to be diffusely reflected and acts as a point source located on the retina. The reflected light passes through the eye and forms a distorted wavefront in front of the eye that results from the ocular aberrations. The aberrated wavefront is then directed to the wavefront sensor.
A point source of radiation on the retina would be ideal for such measurements. However, when the perfect eye receives a collimated beam of light, the best possible image on the retina is a diffraction-limited spot. As illustrated by way of example, with Penny et al. and Liang et al., discussed above, and typical for those of skill in the art, parallel or collimated beams are used with the optics of the eye being measured to achieve this diffraction-limited spot for such objective measurements. To do so requires that a setup for each patient include a corrective lens or lens combination and adjustments thereto for accommodating that patient""s specific visual acuity. Providing a corrective or lens combination, as well as setting up for its use, becomes cumbersome and time consuming, and requires additional expense. Eliminating the need for such corrective optics is desirable and eliminates a variable within optical measurement systems that typically include many variables. Further, there is a need for providing optical characteristics of an eye without requiring feedback from the patient. By way of example, the patient may be a wild or domestic animal, living or dead.
In view of the foregoing background, it is therefore an object of the present invention to provide a refraction measurement system that easily accommodates the measurement of vision characteristics of the eye, even in the presence of finite refractive errors.
It is another object to improve upon the time required for a patient to be in a fixed position during examination, while at the same time providing a useful source of light on the retina of the eye to be measured regardless of the characteristics of the eye of that patient or other patients to be examined.
It is a further object to measure such characteristics without requiring patient or operator feedback.
These and other objects, advantages and features of the present invention, are provided by a method aspect of the invention for measuring optical characteristics of an optical system including the focusing of an optical beam proximate an anterior surface of the optical system, for placing a finite source of secondary radiation on a focal surface of the optical system, which secondary radiation is emitted from the focal surface as a reflected wavefront of radiation that passes through the optical system, projecting the reflected wavefront onto a wavefront analyzer, and measuring characteristics of the optical system associated with the reflected wavefront.
In a preferred embodiment, the method includes the step of measuring defects of the eye, which includes the steps of focusing an optical beam onto an anterior surface of the eye, other than the retina, for providing a finite source of secondary radiation on the retina of the eye, which secondary radiation is emitted from the retina as a reflected wavefront of radiation that passes through the eye, directing the reflected wavefront onto a wavefront analyzer, and measuring distortions associated with the reflected wavefront. A preferred embodiment of the invention includes the step of focusing the projected optical beam on the anterior surface of the cornea. In an alternate embodiment the optical beam is focused behind the retina.
An apparatus for effectively performing such measurements includes means for focusing an optical beam onto an anterior surface of the optical system or eye, other than the retina, for providing a finite secondary radiation source on the focal surface, or retina of the eye, which finite secondary radiation source is emitted from the retina as a reflected wavefront of radiation that passes through the eye, means for directing the reflected wavefront onto a wavefront analyzer, and a wavefront analyzer for measuring distortions associated with the reflected wavefront. In one preferred embodiment of the present invention, a laser beam is focused onto the surface of the cornea with a long-focal-length lens, which converges the beam through a small angle for passing through the iris of the eye and providing a finite secondary radiation source on the retina of the eye, which finite secondary radiation source is emitted from the retina through the optics of the eye as the wavefront to be measured. In an alternate embodiment the apparatus comprises means for focusing the optical beam behind the retina